The present invention relates methods for fabricating a bottom anti reflective coating on semiconductors, and more specifically to determining an anti reflective coating thickness for patterning a thin film semiconductor layer over a high K dielectric layer.
Many silicon devices used in modern integrated circuits utilize a field effect structure that comprises a polysilicon gate positioned over a channel region within a silicon substrate. The gate is separated from the channel region by a dielectric material such as silicon dioxide.
Such a transistor is typically fabricated using lithography techniques that include first growing the silicon dioxide on the surface of the substrate, depositing a polysilicon gate layer over the silicon dioxide, depositing an anti reflective coating over the surface of the polysilicon gate layer, depositing a photoresist layer over the anti reflective coating, exposing the photoresist layer using patterned coherent illumination, developing the photoresist to form a mask, and etching the anti reflective coating and the polysilicon layer to form the gate.
Without the anti reflective coating, the interface between the polysilicon and the photoresist would reflect the patterned illumination and degrade the contrast of the patterned illumination and thereby degrade the precision of the mask and the edge profile of the gate subsequently formed in the polysilicon layer. The anti reflective coating typically absorbs illumination at the lithography wavelength and thereby prevents reflection from the interface of the antireflective coating and the polysilicon layer from degrading the precision of the mask.
Generally the thickness of the silicon layer and the anti reflective properties of the silicon dioxide eliminate any need to consider reflected lithography illumination from the interface of the silicon layer and the silicon dioxide layer.
However, as the size of transistor structures are decreased, there is a trend to decrease the thickness of the silicon gate and to replace the silicon dioxide with other dielectric materials with a dielectric constant greater than that of silicon dioxide. Such thin film silicon layers and high k dielectrics tend to increase the intensity of illumination reflected from the interface between the silicon and the high k dielectric.
Accordingly there is a strong need in the art for a method of fabricating a transistor using an anti reflective coating that is useful with a very thin polysilicon gate layer positioned over a dielectric material with a high dielectric constant.
A first aspect of the present invention is to provide a method of selecting an anti reflective layer thickness for placement between a photoresist layer to be patterned and a polysilicon layer for a device with a high K dielectric layer below the polysilicon layer. The method comprises selecting a trial anti reflective layer thickness and calculating if the trail anti reflective layer thickness will give a total reflected coherent illumination intensity below a predetermined threshold that is low enough to provide sharp contrast when exposing a photoresist. Calculating the total reflected coherent illumination intensity comprises: a) determining a first coherent illumination intensity reflected from an interface between the photoresist layer and the anti reflective layer; b) determining a second coherent illumination intensity reflected from an interface between the anti reflective layer and the polysilicon layer; c) determining a third coherent illumination intensity reflected from an interface between the polysilicon layer and the high K dielectric layer; d) determining a fourth coherent illumination intensity reflected from an interface between the high K dielectric layer and a base substrate below the high K dielectric layer; and e) determining a total reflected coherent illumination intensity that comprises the sum of the first coherent illumination intensity, the second coherent illumination intensity, the third coherent illumination intensity, and the fourth coherent illumination intensity. The trail anti reflective layer thickness as the anti reflective layer thickness if the total coherent illumination intensity is below the predetermined threshold.
The first coherent illumination intensity is equal the intensity of coherent illumination incident on the interface between the photoresist layer and the anti reflective layer multiplied by a first reflection coefficient and phase shifted by Π radians. The first reflection coefficient is equal to the quotient of the difference between the absolute index of refraction of the photoresist layer and the absolute index of refraction of the anti reflection layer divided by the sum of the absolute index of refraction of the photoresist layer and the absolute index of refraction of the anti reflection layer.
The second coherent illumination intensity is equal the intensity of coherent illumination incident on the interface between the photoresist layer and the anti reflective layer that is phase shifted by Π radians plus a phase equal to twice a phase shift that occurs by transmission through the anti reflective layer and the intensity is multiplied by: a) a fraction of illumination transmitted through the anti reflective layer squared, b) a first transmission coefficient squared, and c) a second reflection coefficient.
The first transmission coefficient is equal to the quotient of 4 multiplied by the absolute index of refraction of the photoresist layer multiplied by the absolute index of refraction of the anti reflection layer divided by the square of the sum of the absolute index of refraction of the photoresist layer and the absolute index of refraction of the anti reflection layer. The second reflection coefficient is equal to the quotient of the difference between the absolute index of refraction of the anti reflection layer and the absolute index of refraction of the polysilicon layer divided by the sum of the absolute index of refraction of the anti reflection layer and the absolute index of refraction of the polysilicon layer.
The third coherent illumination intensity is equal the intensity of coherent illumination incident on the interface between the photoresist layer and the anti reflective layer that is phase shifted by Π radians plus a phase equal to twice a phase shift that occurs by transmission through the anti reflective layer plus twice a phase shift that occurs by transmission through the silicon gate layer, and, multiplied by: a) a fraction of illumination transmitted through the anti reflective layer squared, b) a fraction of illumination transmitted through the silicon gate layer squared, c) the first transmission coefficient squared, d) a second transmission coefficient squared, and d) a third reflection coefficient.
The second transmission coefficient is equal to the quotient of 4 multiplied by the absolute index of refraction of the anti reflection layer multiplied by the absolute index of refraction of the silicon gate layer divided by the square of the sum of the absolute index of refraction of the anti reflection layer and the absolute index of refraction of the silicon gate layer. The third reflection coefficient is equal to the quotient of the difference between the absolute index of refraction of the silicon gate layer and the absolute index of refraction of the high K dielectric divided by the sum of the absolute index of refraction of the silicon gate layer and the absolute index of refraction of the high K dielectric.
The fourth coherent illumination intensity is equal the intensity of coherent illumination incident on the interface between the photoresist layer and the anti reflective layer that is phase shifted by Π radians plus a phase equal to twice a phase shift that occurs by transmission through the anti reflective layer, twice a phase shift that occurs by transmission through the silicon gate layer, and twice a phase shift through the high K dielectric layer; and multiplied by: a) a fraction of illumination transmitted through the anti reflective layer squared, b) a fraction of illumination transmitted through the silicon gate layer squared, c) a fraction of illumination transmitted through the high K dielectric layer, d) the first transmission coefficient squared, e) the second transmission coefficient squared, e) a third transmission coefficient squared, and f) a fourth reflection coefficient.
The third transmission coefficient is equal to the quotient of 4 multiplied by the absolute index of refraction of the polysilicon layer multiplied by the absolute index of refraction of the high K dielectric layer divided by the square of the sum of the absolute index of refraction of the polysilicon layer and the absolute index of refraction of the high K dielectric layer. The fourth reflection coefficient is equal to the quotient of the difference between the absolute index of refraction of the high K dielectric layer and the absolute index of refraction of the substrate divided by the sum of the absolute index of refraction of the high K dielectric layer and the absolute index of refraction of the substrate.